Mass spectrometer detector arrays

ABSTRACT

The Mattauch-type mass spectrometer normally records the mass spectrum of a chemical compound on a photographic plate. It would be desirable to replace this plate with an array of electronic detectors whose outputs would be stored for computer analysis. A new class of detector array configuration is proposed which matches the resolution of the array to the inherent resolution of the instrument. In so doing, a 64 percent saving in circuitry and data storage requirements is typically achieved when compared with the requirements of a uniform linear array which is capable of utilizing the full resolution of the instrument over the entire mass scale. This saving accrues when an array 25 cm. long is located in the focal plane of the instrument starting from a point 5 cm. from the mean ion entry point to the magnetic sector of the instrument.

United States Patent Hirschel [451 Jan. 18, 1972 [54] MASS SPECTROMETER DETECTOR ARRAYS [72] inventor: Joel Hirschel, 253 West 72nd St., New

York, N.Y. 10023 2 2 Filed: 0ct.27, 1969 211 Appl.No.: 870,509

[52] US. Cl. ..250/41.9 D [5 1] Int. Cl. ...H0lj 39/34 [58] Field of Search ..250/41.9 D

Primary Examiner-William F. Lindquist [57] ABSTRACT I The Mattauch-type mass spectrometer normally records the A new class of detector array configuration is proposed which matches the resolution of the array to the inherent resolution of the instrument. In so doing, a 64 percent saving in circuitry [56] References Cited and data storage requirements is typically achieved when compared with the requirements of a uniform linear array UNITED STATES PATENTS which is capable of utilizing the full resolution of the instru- 3 240 931 3/1966 Wiley at al 250/41 9 ment over the entire mass scale. This saving accrues when an 3522428 8/1970 Powers "250/4l'9 array 25 cm. long is located in the focal plane of the instrument starting from a point 5 cm. from the mean ion entry FOREIGN PATENTS OR APPUCATIONS point to the magnetic sector of the instrument.

1,100,162 1/1968 Great Britain ..250/41 .9 12 Claims, 4 Drawing Figures P JfifPRtJi/ED DETECTUR ARRAY //V FOCAL PLANE [lfCTZQ/C' /0/V SOURCE WEN-IEO JAN] 8 1922 3.636, 345

saw 1 ur 2 PMENTEB Jun 8 m2 SHEET 2 [IF 2 C3) 6072522222 E Array 7/ my a w 5, g 3 W/// f f 5% V Pea I fc) Dual Verified afidar) Defeczor Arra yf z INVENTOR.

Joel f/z'rtscfiel MASS SPECTROMETER DETECTOR ARRAYS BACKGROUND OF THE INVENTION Among the physical methods is mass spectrometry. In' mass spectrometry, a sample is ionized, the ions are accelerated and subjected to electromagnetic fields and are then detected 'by their impinging on a photographic plate. Analysis of the mass spectrum thus produced then yields the required information on the unknown sample.

Approximately different mass spectra are recorded separately on strips runningthe length of the photographic plate. The plate is removed from the instrument, developed, and then read with a microdensitometer which produces a graph of optical density versus position of the mass spectrum. While good results are obtained in this manner, there. are several inherent drawbacks. The process is time consuming causing delay between experimentiand result. Often, plates are removed after only a few spectra have been recorded thus increasing the cost of operation. Sometimes onespectral line must be severely overexposed in order to record .a much weaker line. This, of course, results in a nonlinear'relation between the optical density and the number of ions striking the plate which is not easily analyzed. Calibration is achieved by introducing a known compound and noting the positions of the known spectral lines.

In 1936, Mattauch presented the theory of a mass spectrometer which was double-focusing simultaneously for all masses (50 Physical Review 6 l 7). The instrument usesa combination of a radial electric field and a homogeneousrnagnetic field to obtain both velocity and direction focusing over the entire focal plane simultaneously for all masses.

The schematic arrangement of the instrumentis shown in FIG. 1.

Qualitatively, ions are formed in a source and accelerated through a potential, V. The ions enter. the instrument through the entrance slit; 8,. They then travel a distance, 1,, where they enter the radial electric field formed between the plates of the coaxial capacitor which forms the electric sector of the instrument. The ions are acted on by the electric field to cause their trajectories to bend in the-same general direction as thecurvature of the plates, however the trajectories of'the higher energy ions are bent less than'those of lowerenergy ions. Because of the values selected for 1,, and di the ions leave the electric sector in parallel rays spread across the exit according to their energies but focused at infinity. Because of the parallelism of the trajectories, the separation between the exit boundary of the electric sector and the entrance boundary of the magnetic sector may be arbitrarily chosen.

When two ions of the same mass, but different velocities, enter the magnetic sector, they enter perpendicular to the boundary, but separated in space by the prior action of the electric sector. The magnetic field is uniform anddirected normal to the plane of motion of the ions. The trajectories of thesetwo ions are both circular, but-with different radii. The higher energy ion is bent into an orbit with a larger radius than the lower energy ion and they both strikethe-same point in the focal plane. Ions of larger mass will strike the detector further down the focal plane with the same focusing action.

Slit S,, in combination with the distance 1,, actsto control the angles the ion trajectories may-make with'the entrance boundary to the electric sector. In the limit, as S, approaches zero, the ions appear to emerge from a point source, or more correctly, a line source located at 3,. As S, gets larger, the effective source point becomes semi-indeterminate ranging between the location of S, and some point behind it. This then causes a departure from the desired parallelism of the trajectories leaving the electric sector and produces a deleterious effect on the resolution of the instrument.

The presence of slit 8, is not required by the theory, but is .simply for long term protection. It reduces the number of ions entering the electric sector which would not exit, but would v.accumulate on the sector walls. This accumulation would :eventually cause a distortion of the electric field and degrade the focusing qualities of the instrument.

Slit S limits the energy range of the ions entering the magnetic sector. This is necessary because the Mattauch-type mass spectrometer is double-focusing for all masses only to first order.

Mattauch showed that the theoretical resolution of the instrument may be expressed by (Am/m) (2 S,/r (1) that is, the smallest fractional mass difference which can be resolved is dependent only on slit width S, and the dimensions of the electric sector.

In 1947 Shaw and-Rall showed that (Am/m) cu /p) (2) where p is the distance from the. mean entry point into the magnetic sector to the point of detection in the focal plane; and .Ap is the smallest detectable separation between spectral lines 18 Review of Scientific Instruments 278).

SUMMARY OF THE INVENTION In order to obtain more effective use from the mass spectrometer, it would be desirable toreplace the present detection scheme with an electronic scheme which will extract as much information as the photographic plate method, but will eliminate the time-consuminglprocessing operation. Ideally,

such a scheme will make provision for an interface with a computerfor quick, accurate processing of the collected data.

Therefore, an object of the invention is to provide an array of discrete detector elements having a resolution matched to the resolution of the instrument thereby minimizing the number of elements.

Another object is to provide an array which insures that no spectral line may escape detection, no matter how narrow.

Another object is to provide a data collection and storage system for interfacing with a computer which requires a minimum amount of auxiliary circuitry.

Another object is to disclose detector arrays which are particularly well suited to take advantage of integrated circuit and thin film techniques in their manufacture.

Another object is to disclose detector arrays which are particularly well suited for computer controlled manufacturing methods.

These objects are achieved in my invention by making the width of each detector element in the array a function of position in the focal plane. Asa result, the resolution available from the arrayrrnay .be matched tothe resolution of the spectrometer thereby reducing to a'minimum, the number of detector elements and associated circuitry.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 shows the arrangement of a typical Mattauch-type mass spectrometer.

FIG. 2a is a face-on view of auniform linear detector array for comparison purposes. v

FIG. 2b is a face-on view of one illustrative embodiment of a constant resolution array.

FIG. 2c is a face-on view of a detector array which insures that no spectral line may escape detection.

DETAILED DESCRIPTION For simplicity, let us define a quantity, R=m/Am, so that we may speak of the resolution as being one part in R, or more simply, as justR. When using-R, however, it must be carefully noted whether what is meant is the resolution of the instrument or the resolution available from the detector array.

Let us first consider a structure composed of a uniform linear array of detectors. The present state of the art will allow element dimensions no smaller than about 1 micron meters), so let us assume an equal length for the width of each detector and the associated space which separates it from the adjacent detector as shown in FIG. 2a. Also, the width of each detector space pair, 6, is the same in any region along the array. This means that the width of each detector is 8/2 and the width of each space or isolating region separating adjacent detectors is also 8/2.

Furthermore, let us establish a criterion for the resolution of the array whereby the minimum detectable separation between adjacent peaks in the mass spectrum is stated in terms of element lengths, namely as K8. The preferable value of K for the array of FIG. 2a is"l.5. For a greater margin of safety a larger valuefof K may be selected. Substituting the value of 1.58 for the minimum detectable Ap, and R for m/Am in Eq.(2), we find that R=p/( 38) for the uniform linear array. Since in this array 8 isa constant, we see that the resolution increases with distance along the array.

However, from Eq.(1) it is apparent that the resolution available from the instrument itself is independent of position in the focal plane. Therefore, if a uniform linear array is capable of resolution equal to that of the instrument at the beginning of the array, it is capable of greater resolution than is the instrument itself over the remainder of the array. This means that more detector elements and associated circuitry are provided than are able to be made use of efficiently. The number of elements is equal to R for an array 25 cm. long starting at p=5 cm.

When the instrument is in use, slit width S is fixed as is r, so the resolution of the instrument, R, may be treated as a constant. To design an efficient array the resolution available from the array must be matched to the resolution of the instrument. As shown in FIG. 2b, such an array to provide resolution independent of p requires that the width of each detector space pair (5,.) be proportional to p. ln the illustrative embodiment shown, 5n=p,,/( 3R), the width of each detector is 8,,l2 and the width of the associated space is also 8,,l2. The number of elements in this array is 5.375R for an array 25 cm. long starting at p=5 cm., a saving of 64.2 percent over the uniform linear array. Of course, in general, the width of each detector may be M8,, and the width of the associated space 14418,, where 0 M l. in the example cited, M=0.5.

The width of each detector space pair, 8, may be calculated in an iterative manner particularly well suited to computer controlled manufacturing techniques. Specifically 5,=(8l/2KR) (3) where p is the location of the beginning of the array; and the width of the 11'' pair is A problem may arise when the foregoing array is used to detect very narrow spectral lines. If one of these extremely narrow lines should fall in the space separating two adjacent detectors it may not be detected. To preclude this possibility, a dual version of the Constant R array may be used as shown in FIG. 20. The Dual Constant R array may take one of two almost identical forms.

One form may be described conceptually by starting from the Single Constant R array of FIG. 2b where M=0.5. First a narrow strip for separating detector elements is extended longitudinally across the center of the array. This then leaves two detectors, one above the other, in each element of length, 8,. Next, shift either the top or bottom rows of detector elements so that each detector in that row occupies the location formerly occupied by the separating space. The result is shown in FlG. 2c and it may be seen that there is no location along the array which is not served by a detector. For this array, the

preferred value is K=l for determining the minimum detectable 8p and for use in Eqs.(3) and (4).

Almost identical to the foregoing array is another version of the Dual Constant R array which more closely matches the instrument resolution. In the foregoing version two successive detectors are of equal width. The preferred embodiment of the invention contains detectors, each having a width given by Eqs.(3) and (4) where, however, 8,, represents the width of the n' individual detector (21,22), rather than a detector space pair as before, and the preferred value of K is 2.

An advantage of the constant resolution array is that a particular given spectral line falling anywhere along the focal plane as a result of the values of the electric and magnetic fields chosen, will result in the same output from a detector in the same position relative to the position of that line. This is true because as a spectral line is caused to sweep from the beginning to the end of the focal plane, its width increases. The widths of the detectors along the Constant R array increase by the same amount. Of course, the field values in the instrument are held fixed while in use with either the photographic plate or the array detection schemes.

The replacement by a Constant R detector array of the photographic plate in the Mattauch-type mass spectrometer results in an improved instrument in that its utility to the user is enhanced by achieving more rapid and accurate results through an interface with a suitably programmed computer. The Constant R detector array serves as theheart of a data collection and storage system.2 l25 which eliminates the inaccuracies and delays of the conventional approach while minimizing the number of detector elements and associated circuitry.

The detector arrays described may be manufactured by any of a number of known techniques. Each detector must have an output proportional to the number of ion impacts or to the added charge to its surface area. This may be accomplished by using a layer-structured semiconductor plate and etching away the areas to be deactivated by chemical, laser or electron beam techniques or, alternatively, may be built up by electron beam deposition methods which have achieved element dimensions as small as 1 micron as described in an article by Landauer and Hall Science 740).

Connection between the detectors and their associated circuitry 24,25 may be through deposited metal film contacts 23 or by electron beam scanning as described by Crowell and Labuda in an article entitled, The Silicon Diode Array Camera Tube" in the May-June 1969 issue of the Bell System Technical Journal, (48 B.S.T.J. 1481).

Variations on the basic mass spectrometer, such as removing the focal plane from the region of high-magnetic fields through the application of well known principles of ion optics, is recognized. The modifying effects of such variations may be compensated for in my invention by those skilled in the art without departing from the spirit and scope of the invention.

What is claimed is:

l. A focal plane structure for use in a mass spectrometer including a focal plane, a magnetic sector and an ion entry region to the magnetic sector, which comprises:

a plurality of ion responsive detector elements; and

means for supporting the detector elements in an array for placement in the focal plane of the spectrometer;

the lengths of the detector elements extending transverse to the longitudinal axis of the array;

the width of each detector element being linearly proportional to its distance from a point corresponding to the mean ion entry point of the ion entry region of the magnetic sector of the spectrometer; and adjacent detector elements being separated by an isolating region.

2. A structure in accordance with claim 1, wherein each detector element extends transversely across the focal plane; each detector element forming a unit with an adjoining space separating adjacent detectors in the array, the width of each unit being linearly proportional to its distance from the mean ion entry point into the magnetic sector of the spectrometer; and the width of each detector being a fixed percentage of the width of its unit.

3. A structure in accordance with claim 2, wherein the constant of proportionality for the width of each unit is (l/3R), where R is a constant equal to the resolution of the spectrometer.

4. A structure in accordance with claim 1, wherein successive detector elements extend .altemately in opposite directions from the longitudinal axis of the array, adjacent detectors on one side of the longitudinal axis being separated from each other by a substantially inactive region which is opposite a detector element on the other side of the longitudinal axis of the array.

5. A structure in accordance with claim 4, wherein the constant of proportionality for the width of each detector element is l/4R, where R is a constant equal to the resolution of the spectrometer.

6. An improved mass spectrometer including a focal plane and a magnetic sector having a boundary, a segment of which boundary forms an ion entry region, wherein the improvement comprises:

a plurality of detector elements for generating an output in response to ion impacts; and

means for supporting the detector elements in the focal plane of the spectrometer in an array;

the lengths of the detector elements extending transverse to the longitudinal axis of the focal plane; the width of each detector element being linearly proportional to its distance from a junction point between the ion entry region boundary segment of the magnetic sector and a line extending through the focal plane; and

adjacent detector elements being separated by a substantially inactive region.

7. An improved mass spectrometer in accordance with claim 6, wherein each detector element extends transversely across the focal plane; each detector element forming a unit with an adjoining inactive region separating adjacent detectors in the array, the width of each unit being linearly proportional to its distance from said junction point; and the width of each detector being a fixed percentage of the width of its unit.

8. An improved mass spectrometer in accordance with claim 6, wherein successive detector elements extend alternately in opposite directions perpendicularly from the longitudinal axis of the focal plane, adjacent detectors on one side of the longitudinal axis being separated from each other by a substantially inactive region located in opposing relationship to a detector element on the other side of the longitudinal axis.

9. A data collection system for a mass spectrometer including a focal plane, which comprises:

a plurality of ion responsive detector elements;

means for supporting the detector elements in an array for placement in the focal plane of the spectrometer; the lengths of the detector elements of the array extending transverse to the longitudinal axis of the array, the width of each detector element being linearly proportional to its distance from a point on a line extending through and beyond the focal plane, and adjacent detector elements being separated by an isolating region;

means for collecting data from the detector elements; and

means for storing the collected data.

10. A data collection system in accordance with claim 9, wherein each detector element extends transversely across the focal plane, each detector element forming a unit with an adjoining inactive element separating adjacent detectors in the array, the width of each unit being linearly proportional to its distance from said point, the width of each detector being a fixed percentage of the width of its unit.

11. A data collection system in accordance with claim 9, wherein successive detector elements extend in alternate directions perpendicularly from the longitudinal axis of the focal plane, adjacent detectors on one side of the longitudinal axis being separated from each other by an isolating region located in opposing relationship to a detector element on the other side of the longitudinal axis.

12. In combination, elements of a mass spectrometer comprising:

a magnetic sector;

an ion entry region of the magnetic sector;

a mean ion entry point in said ion entry region;

a focal plane located in focused relation to the magnetic sector;

a plurality of ion responsive detector elements; and

means for supporting the detector elements in the focal plane in an array;

the array being characterized by the detector elements extending transverse to the longitudinal axis of the focal plane, the width of each detector element being linearly proportional to its distance from the mean ion entry point, and the detector elements being separated by an isolating region. 

1. A focal plane structure for use in a mass spectrometer including a focal plane, a magnetic sector and an ion entry region to the magnetic sector, which comprises: a plurality of ion responsive detector elements; and means for supporting the detector elements in an array for placement in the focal plane of the spectrometer; the lengths of the detector elements extending transverse to the longitudinal axis of the array; the width of each detector element being linearly proportional to its distance from a point corresponding to the mean ion entry point of the ion entry region of the magnetic sector of the spectrometer; and adjacent detector elements being separated by an isolating region.
 2. A structure in accordance with claim 1, wherein each detector element extends transversely across the focal plane; each detector element forming a unit with an adjoining space separating adjacent detectors in the array, the width of each unit being linearly proportional to its distance from the mean ion entry point into the magnetic sector of the spectrometer; and the width of each detector being a fixed percentage of the width of its unit.
 3. A structure in accordance with claim 2, wherein the constant of proportionality for the width of each unit is (1/3R), where R is a constant equal to the resolution of the spectrometer.
 4. A structure in accordance with claim 1, wherein successive detector elements extend alternately in opposite directions from the longitudinal axis of the array, adjacent detectors on one side of the longitudinal axis being separated from each other by a substantially inactive region which is opposite a detector element on the other side of the longitudinal axis of the array.
 5. A structure in accordance with claim 4, wherein the constant of proportionality for the width of each detector element is 1/4R, where R is a constant equal to the resolution of the spectrometer.
 6. An improved mass spectrometer including a focal plane and a magnetic sector having a boundary, a segment of which boundary forms an ion entry region, wherein the improvement comprises: a plurality of detector elements for generating an output in response to ion impacts; and means for supporting the detector elements in the focal plane of the spectrometer in an array; the lengths of the detector elements extending transverse to the longitudinal axis of the focal plane; the width of each detector element being linearly proportional to its distance from a junction point between the ion entry region boundary segment of the magnetic sector and a line extending through the focal plane; and adjacent detector elements being separated by a substantially inactive region.
 7. An improved mass spectrometer in accordance with claim 6, wherein each detector element extends transversely across the focal plane; each detector element forming a unit with an adjoining inactive region separating adjacent detectors in the arraY, the width of each unit being linearly proportional to its distance from said junction point; and the width of each detector being a fixed percentage of the width of its unit.
 8. An improved mass spectrometer in accordance with claim 6, wherein successive detector elements extend alternately in opposite directions perpendicularly from the longitudinal axis of the focal plane, adjacent detectors on one side of the longitudinal axis being separated from each other by a substantially inactive region located in opposing relationship to a detector element on the other side of the longitudinal axis.
 9. A data collection system for a mass spectrometer including a focal plane, which comprises: a plurality of ion responsive detector elements; means for supporting the detector elements in an array for placement in the focal plane of the spectrometer; the lengths of the detector elements of the array extending transverse to the longitudinal axis of the array, the width of each detector element being linearly proportional to its distance from a point on a line extending through and beyond the focal plane, and adjacent detector elements being separated by an isolating region; means for collecting data from the detector elements; and means for storing the collected data.
 10. A data collection system in accordance with claim 9, wherein each detector element extends transversely across the focal plane, each detector element forming a unit with an adjoining inactive element separating adjacent detectors in the array, the width of each unit being linearly proportional to its distance from said point, the width of each detector being a fixed percentage of the width of its unit.
 11. A data collection system in accordance with claim 9, wherein successive detector elements extend in alternate directions perpendicularly from the longitudinal axis of the focal plane, adjacent detectors on one side of the longitudinal axis being separated from each other by an isolating region located in opposing relationship to a detector element on the other side of the longitudinal axis.
 12. In combination, elements of a mass spectrometer comprising: a magnetic sector; an ion entry region of the magnetic sector; a mean ion entry point in said ion entry region; a focal plane located in focused relation to the magnetic sector; a plurality of ion responsive detector elements; and means for supporting the detector elements in the focal plane in an array; the array being characterized by the detector elements extending transverse to the longitudinal axis of the focal plane, the width of each detector element being linearly proportional to its distance from the mean ion entry point, and the detector elements being separated by an isolating region. 